1. Field on the Invention
The present invention relates to the measurement of electromagnetic absorption of Radar Absorbing Material (RAM) and more particularly to the measuring of surface traveling wave attenuation.
2. Description of the Related Art
The radar cross section (RCS) due to a surface traveling wave is significant when a long smooth object is illuminated by electromagnetic energy at relatively low angles of incidence (near grazing angle). The traveling wave is launched only if there is a component of the incident electric field tangential to tile surface and in the plane of incidence. The RCS pattern of a long thin structure is discussed in the book entitled "Radar Cross Section," authored by E. F. Knot et al, 1985, pages 147-150 which references the article entitled, "End-Fire Echo Area of Long, Thin Bodies," IRE Trans. Antennas Propag., Vol. AP-6, No. 1, Jan. 1958, pages 133-139 authored by L. Peters, Jr. In this article, a long thin structure is approximated by a thin wire.
Referring now to the figures, FIG. 1 is a reproduction of FIG. 5-12 from the book entitled "Radar Cross Section", with numeral designations added for clarity. The scattering 8 from a thin wire 10 excited by a plane wave 12 is shown in this figure. There exists two current waves, one traveling in the forward direction and one in the backward direction (14 and 16, respectively). The backward traveling current wave 16 will give rise to the same kind of RCS pattern generated by the forward current wave 14, but its location in space will be in the opposite direction. Due to the impedance mismatch at the end of the wire 10 and finite conductivity at the wire surface, the level of the scattering in the backward direction will be less than that in the forward direction.
The backscattered RCS from tile backward current wave in a long, smooth metallic surface is the quantity of interest since the energy is directed back to the radar antenna for the detection of the target. Although the above thin wire analogy assumes a long slender scatterer, the surface wave phenomenon occurs in other structures such as airfoils and missile bodies. In fact, any discontinuity due to the termination of a finite structure or surface discontinuity of a subsection of a larger surface due to seams and gaps can cause this type of scattering.
The maximum RCS of the surface traveling wave for a long slender body is located at the angle approximated by: EQU .theta.=49.35 (.lambda./b).sup.1/2 ( 1)
where .theta. is the angle (in degrees) from the long axis of the structure, .lambda. is the wavelength of tile electromagnetic wave, and b is the length of the body. This location of the first surface traveling wave lobe is important since it has tile highest level of backscatter to the radar receiver.
The suppression of the traveling wave scattering is typically provided by bonding magnetic surface absorbers (magnetic RAM) to the part of the structure that supports the traveling wave. Determination of the effectiveness of the magnetic RAM in suppressing the traveling wave is performed by measuring the RCS of a full scale model of a long thin target that supports the traveling wave. Full scale models are required for this type of measurement since the magnetic RAM material is frequency sensitive. In the UHF and VHF band, where the electrical wavelength is very long, the target can be quite large and the measurement has to be performed in a very large anechoic chamber or outdoor RCS range. The problems associated with such measurement are the high cost of fabricating the model and the rapid roll off of the low frequency signal in the free space environment. In order to reduce the influence of background noise and reflection from other structures (except the target), the time domain reflected signal is processed by a gating procedure to exclude noise and other returns. The limitation to such a technique is that some of the large signals that are outside the range gate but within the digitizer time window is present in the receiver due to the antenna mismatch. The unwanted signal limits the amount of gain for the receiver. This limits the resolution of the signal reflected from the target and usually results in a dynamic range of only 10 to 15 dB.
The limitations described above are eliminated by the use of a parallel plate system for the RCS measurement for two dimensional targets. In this system, the electromagnetic energy is confined between the parallel conducting plates. Since the signal currents are conducted through the conducting skins of the parallel plates the low frequency roll off is extremely small. The parallel plates also offer excellent shielding from other electromagnetic signals that interfere with the outdoor or anechoic ranges.
The common type of measurement in a parallel plate is the specular RCS from a 2 dimensional structure. A section of the 2 dimensional target is placed in the quiet zone of the parallel plate system. This section of the test specimen must have a thickness such that when placed in the parallel plate two flat sides of the specimen are in physical contact with the top and bottom plates. The electromagnetic fields in the parallel plate system comprise plane transverse electromagnetic plane waves, each with a vertically polarized election field (i.e. the direction of the electric field is from the bottom plate to the top plate). Since a traveling wave occurs only when the electric field is in the plane of incidence, an alternative setup had to be developed to evaluate the traveling wave attenuation capability of magnetic materials while using a parallel plate setup.